// run: $exec < input
#include <iostream>
#include <iomanip>
#include <algorithm>
#include <cmath>

int a[20];
int n;

double area(double x, double y, double z)
{
	double p = (x + y + z) / 2.0;
	return sqrt(p * (p - x) * (p - y) * (p - z));
}

int main()
{
	std::ios::sync_with_stdio(false);
	while (std::cin >> n && n) {
		for (int i = 0; i < n; i++) std::cin >> a[i];
		std::sort(a, a + n);
		double ans = 0;
		for (int i = n - 1; i >= 2; ) {
			int x = a[i], y = a[i - 1], z = a[i - 2];
			if (y + z > x) {
				ans += area(x, y, z);
				i -= 3;
			} else i--;
		}
		std::cout << std::fixed << std::setprecision(2) << ans << "\n";
	}
}

